Documentation

In cabal a self dependency probably means the library is needed
while building the executables. In debian it would mean that the
package needs an earlier version of itself to build, so we use this
to filter such dependencies out.

Function that applies the mapping from cabal names to debian
names based on version numbers. If a version split happens at v,
this will return the ltName if < v, and the geName if the relation
is >= v.